Often when using navigation my screen would dim then later brighten again then dim again, etc. This happened with automatic screen brightness turned off in my Bionic's display settings. This is an annoyance that has been fixed with the latest update of Google Maps.
If you have had this problem, after getting the latest Google Maps update -
To disable screen dimming during navigation -
To access this new setting initiate a navigation session, tap the menu button, then select More > Settings and de-select Screen Dimming (it's on by default).